In the fatigue design of steel catenary risers, there are concerns regarding the fatigue damage to girth welds from low stresses, below the constant amplitude fatigue limit, in the loading spectrum and the validity of Miner's cumulative damage rule. These fundamental issues were addressed in a recent joint-industrial project (JIP). A key feature was development of the resonance fatigue testing rigs to enable them to test full-scale pipes under variable amplitude loading. Such tests were performed under a loading spectrum representative of that experienced by some risers, with many tests lasting over 100 million cycles to investigate the fatigue damage due to small stresses as well as the validity of Miner's rule. However, the resonance rigs are only capable of producing spectrum loading by gradually increasing or decreasing the applied load whereas more “spiky” random load sequences may be relevant in practice. Therefore, the program also included fatigue tests in conventional testing machines on strip specimens cut from pipes to compare the two types of loading sequence. This paper presents the results of these tests, conclusions drawn, and recommendations for changes to current fatigue design guidance for girth welded pipes regarding the definition of the fatigue limit, allowance for the damaging effect of low stresses, and the validity of Miner's rule.
Fatigue Testing of Full Scale Girth Welded Pipes Under Variable Amplitude Loading
Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received January 23, 2013; final manuscript received October 15, 2013; published online January 17, 2014. Assoc. Editor: Xin Sun.
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Zhang, Y., and Maddox, S. J. (January 17, 2014). "Fatigue Testing of Full Scale Girth Welded Pipes Under Variable Amplitude Loading." ASME. J. Offshore Mech. Arct. Eng. May 2014; 136(2): 021401. https://doi.org/10.1115/1.4026025
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